IMCs Microstructure Evolution Dependence of Mechanical Properties for Ni/Sn/Ni Micro Solder-Joints

The current miniaturization trend of microelectronic devices drives the size of solder joints to continually scale down. The miniaturized joints considerably increase intermetallic compounds (IMCs) volume fraction to trigger mechanical reliability issues. This study investigated precise relationships between varying IMC volumes and mechanical properties of Ni/Sn(20μm)/Ni micro-joints. A designed method that followed the IMC volume as the only variable was used to prepare micro-joint samples with different IMC volumes. The continuously thickened Ni₃Sn₄ IMCs exhibited a noticeable morphology evolution from rod-like to chunky shape. The subsequent tensile tests showed unexpected tensile strength responses as increasing Ni₃Sn₄ volume, which was strongly associated with the Ni₃Sn₄ morphological evolutions. Fractographic analysis displayed that the ductile fracture dominates the 20%-40% IMC micro-joints, whereas the brittle fracture governs the 40%-80% IMC micro-joints. For the ductile fracture-dominated joints, an abnormal reduction in strength occurred as increasing IMCs volume from 20% to 40%. This is primarily due to severe stress concentrations caused by the transformed long rod-typed morphology of the Ni₃Sn₄. For the brittle fracture-dominated joints, the strength appeared a monotonous increase as the Ni₃Sn₄ volume increased. This may be attributed to the increased crack resistance resulting from continuous coarsening of the chunky Ni₃Sn₄ without any voids. Moreover, the finite element analysis was provided to further understand the joint failure mechanisms.